1. Field
The present application generally relates to optical metrology of a structure formed on a semiconductor wafer, and, more particularly, to optical metrology of multiple patterned layers.
2. Description of the Related Art
Optical metrology involves directing an incident beam at a structure, measuring the resulting diffracted beam, and analyzing the diffracted beam to determine a feature of the structure, such as a critical dimension (CD), profile, and the like. In semiconductor manufacturing, optical metrology is typically used for quality assurance. For example, after fabricating a test structure, such as a grating array, in a test pad, in proximity to a corresponding structure in a die, an optical metrology system is used to determine the profile of the test structure. By determining the profile of the test structure, the quality of the fabrication process utilized to form the test structure, and by extension the corresponding structure proximate the test structure, can be evaluated.
When multiple patterned layers are measured using conventional optical metrology, the presence of the bottom patterned layer below the top patterned layer can result in inaccuracy. For example, the presence of the bottom patterned layer can increase the amount of the incident beam that is absorbed, which reduces the strength of the measured diffraction signal. Additionally, the increased number of profile parameters needed to characterize the multiple patterned layers increases the potential for correlated or insensitive profile parameters.